The double-decker aircraft pioneered the use of the term "jumbo jet." But the "Queen of the Skies" is growing old, and its four engines are thirstier than newer, more fuel-efficient jets.

The airlines, however, disagree; already they are turning their minds toward tomorrow’s jumbo jets. And in their visions there are many-splendored pleasure domes that even an oil sheik could envy.

In January, United Airlines announced that it will expedite the retirement of its fleet of Boeing 747-400 jumbo jets. United, the last major airline in the US to operate the venerable icon ...